Process for the electrolytic formation of aluminum coatings on metallic surfaces in molten salt bath

ABSTRACT

Metallic surfaces are electrolytically coated Aluminum aluminum in a molten salt bath containing aluminum chloride. Aluminum metal is used as an anode and a metallic surface as a cathode. Hydrogen ions are supplied to the bath from an external source during electrolysis. The metallic surface is electroplated with a metal having a higher hydrogen overvoltage than that for aluminum prior to plating aluminum thereon.

United States Patent [72] Inventors Akira Miyata Tokyo; Chikayoshi Tomita, Tokyo; Akio Suzuki, Tokyo; Hideyo Okubo, Kanagawa-ken; Masahiko Nagakuni, Kanagawa-ken, all of Japan [21] Appl. No. 828,050

[22] Filed Mar. 24, 1969 v [23] Division of Ser. No. 507,034,

Nov. 9, 1965, Pat. No. 3,480,521

[45] Patented Oct. 26, 1971 [73] Assignee Nippon Kokan Kabushiki Kaisha Tokyo, Japan [32] Priorities Nov. 13, 1964 [33] Japan Feb. 24, 1965, Japan, No. 40110330; Feb. 24, 1965, Japan, No. 40/103151 [54] PROCESS FOR THE ELECTROLYTIC FORMATION OF ALUMINUM COATINGS ON METALLIC SURFACES IN MOLTEN SALT BATH 7 Claims, No Drawings [52] US. Cl 204/39, 29/196, 29/196.4, 29/196.5, 29/196.6, 29/197 [51] lnt.Cl C23b 5/22, C23b 5/50 [50] Field of Search 204/39, 40, 41, 67; 29/197 [5 6] References Cited UNITED STATES PATENTS 527,846 10/1894 Waldo et al 204/67 2,682,101 6/1954 Whitfield et a1. 204/39 UX 2,752,303 6/1956 Cooper 204/64 2,957,782 10/1960 Boller 204/39 X 3,288,689 11/1966 Azuma 204/39 X Primary Examiner-G, L. Kaplan Altorney-Flynn & Frishauf ABSTRACT: Metallic surfaces are electrolytically coated Aluminum aluminum in a molten salt bath containing aluminum chloride. Aluminum metal is used as an anode and a metallic surface as a cathode. Hydrogen ions are supplied to the bath from an external source during electrolysis. The metallic surface is electroplated with a metal having a higher hydrogen overvoltage than that for aluminum prior to plating aluminum thereon.

PROCESS F OR THE ELECTROLYTIC FORMATION OF ALUMINUM COATINGS ON METALLIC SURFACES IN MOLTEN SALT BATH This is a 9, 1965, of application Ser. No. 507,034, filed Nov. 9,1965, now U.S. Pat. No. 3,480,521.

This invention relates to a process for coating steel, copper and other metallic surfaces with aluminum as protecting films through the way of molten salt electrolysis is a fused bath containing aluminum halide and the like.

It is commonly known to those skilled in the art that various and considerable drawbacks are encountered for carrying out this kind of electrolytic process. Among others, a speedy and unavoidable deterioration of the electrolyte was encountered in the course of repeated electrolysis. This kind of deterioration will invite overall or localized loose deposit of aluminum coating on the metallic surface to be protected, prevent favorable higher cathodic current density from being adopted or result even in a formation of trees or sluglike nontight deposits on the metallic surface. In extreme cases, the coating will represent powder-like appearance and is highly liable to be sealed off even in the course of the water cleaning stage which is conventionally carried out in direct succession to the electrolytic coating process.

For avoiding these drawbacks, addition of various salts such as those of lead, cadmium, chromium and/or the like, has hitherto been proposed and employed. Inclusion of lead and the like other metals, as high as 0.4 percent, in the aluminum coating, resulted however in a lowered anticorrosive performance, an accelerated scale-off tendency, and a rough touch of the coated aluminum film.

it is therefore the main object of the invention to provide an improved process for electrolytically coating steel and other metallic surfaces with aluminum in a fused salt chloride bath, providing and assuring a substantially extended durable life of the bath adapted for the electrolysis.

Another object of the invention is to provide a process of the above kind, capable of providing aluminum-coated metallic stock electrolytically treated.

Still another object is to provide a process of the kind above referred to, capable of providing aluminum-coated metallic products having superior surface conditions of the coating thus produced.

Still further object is to provide a molten salt electrolytic process capable of being carried into effect with ease of operation at a high operating efficiency.

These and further objects, features and advantages of the invention will appear more specifically and clearly as the description proceeds.

ln the improved process as proposed by the present invention, hydrogen ions are supplied in either or both positive and negative sense while the process proceeds. There are numerous ways for carrying out the process. As a representative way to positively supply the hydrogen ions, electrochemically ionized hydrogen, for instance, by contact with platinum black and/or hydrogen chloride gas may be fed directly to the electrolytic bath.

Superior results obtainable by the process of this invention can be attributable to the fact that the aluminum as deposited is loaded with hydrogen ion in a direct or an indirect manner.

An additional or alternative way for the supply of hydrogen ions is to provide prior to the electrolysis a very thin aqueous film on the metallic surface to be aluminum coated, and only then subject the stock to the coating electrolysis afterwards. The precise mechanism, why hydrogen ions can be supplied to adopting such measure, is not clear to us at the present moment. It can be assumed with a certain degree of probability that the aqueous content of the preliminarily formed and maintained film will react with AlCl contained in the molten salt bath:

In this way, hydrogen ions may be supplied to the electrolytically reacting zone in the course of the coating process. As the wetting solution, aqueous acetone solution of any desired concentration may preferably be used. According to our experiments, superior results may be obtained when the aqueous wetting solution contains, solely or in combination, inorganic and/or organic substances or chemicals which have a high affinity to water. For this purpose, methyl and/or ethyl alcohol, ethylene glycol, glycerine, lactic acid and/or formic acid may be employed.

Starch, sugar, casein, egg albumin, gelatin, water glass or the like organic and inorganic substances may be utilized for the same purpose, in the form of syrup or paste.

Various inorganic compounds such as sodium chloride, magnesium chloride, potassium chloride, calcium sulfate, magnesium perchlorate, caustic soda, calcium chloride, aluminum chloride, barium chloride, zinc chloride, chromium chloride, titanium tetrachloride, and the like may also be used in the form of aqueous or alcoholic solution for the same purpose. When necessary, conventional surface active agents, for instance, soap, various known organic derivatives such as sodium salts of high molecular alkyl sulfates, or sulfonates may be used. These agents may be used in the form of a dilute aqueous or alcoholic solution for the purpose of this invention.

Even when these substances are dried up, they will contain a slight amount of water; thus in effect, they are in the form of an aqueous film which can be utilized, according to this invention, in the aforementioned manner.

in the art of molten salt electrolysis for the formation of aluminum coatings, an aluminum mass is frequently submerged in the fused bath so as to act as an anodic electrode, and to replenish the consumed quantity of aluminum component in the bath as the electrolytic process proceeds. The mass dissolves out gradually in the molten hot bath regardless of conducting the electrolytic current and in excess of the consumed quantity of aluminum in the bath. This superfluous solute is liable to be reduced to metallic aluminum which is highly unstable and will act adversely on the desired electrolytic formation of aluminum coating, as will be specifically described hereinafter by way of experiment set forth in example 3.

As the unstable metal aluminum appears in the bath in the above-mentioned manner, it is oxidized to its ionized state in the presence of hydrogen ions as supplied according to the main feature of this invention, as in such case metallic aluminum is liable to be oxidized in the presence of hydrogen ions when hydrochloric acid is supplied for contact therewith. In this way, the aforementioned drawbacks adversely affecting the ideal electrolytic formation of aluminum coating may effectively be obviated relying upon the novel teaching of the present invention.

In the inventive process, the supply of hydrogen ions to the electrolytically reacting zone can be carried out in a negative sense in the following way:

More specifically, the metal stock is preparatorily and cathodically subjected to an electrolyte treatment in the presence of an acid solution containing cations of those metals which have a higher hydrogen overvoltage than that for aluminum such as Pb, Sn, Ti, Zn, Bi, Cr, Cd and/or the like. By this preparatory treatment, these metals are deposited on the stock and carried by the stock to the electrolytically reacting zone in the next succeeding electrolytic aluminum coating process. When the latter process is carried out in the presence of these metals, hydrogen ions contained in the fused salt bath, especially those existing in close proximity of the electrolytically reacting zone, more specifically in the neighborhood of the metallic surface to be subjected to the aluminum coating process, are brought into retardation in the transformation into hydrogen molecules. Although this retarding period is short, it is enough to carry out the electrolytic aluminum coat ing process under favorable conditions. In this way, hydrogen ions may be supplied in a negative sense to the reacting zone. without relying upon any supply source outside of the fused salt bath.

EXAMPLE 1.

Using a molten bath, 200 cc., comprising 60 mole percent of aluminum chloride and 40 mole percent of sodium chloride, a steel stock (Xl5X0.2 mm.) was electrolytically treated as cathode in a conventional manner. Below the steel stock a length of approximately cm. of coiled platinum wire (0.5 mm. thick and 10 mm. coil diameter) was held as anode under tension at a distance of 2 cm. while being submerged in the bath. The cathodic current density was 2 aJdrn. and the EXAMPLE 2.

For comparison, argon gas again in the form of small bubbles was introduced instead of hydrogen under the same operating conditions, and at the same feeding rate as before and throughout the whole electrolytic period which was again 10 minutes. An aluminum coating was produced on the steel stock with no appreciable improvement in the quality of the coating. Although agitation of the bath was utilized, no appreciable gain in the results were achieved, because of the lack of hydrogen ion.

EXAMPLE 3.

1,180 grams of aluminum chloride and 320 grams of sodium chloride were mixed together thoroughly, heated to melting temperature, charged in a glass vessel and kept at a temperature of 160 C., whereupon the vessel was sealingly closed off from the ambient atmosphere. Before the bath was sealed, a sheet of aluminum plate (50Xl50X0.2 mm.) was arranged in the melt as anode and a piece of steel sheet having the same dimensions was provided as cathode. Current was conducted thereto as a cathodic density of 2 a./dm at 0.4 volt for 10 minutes.

On the other hand, a length of coiled aluminum wire, weighing 44.9408 grams, was immersed in the same bath, while continuing the electrolysis, and measurements were made at several time intervals so as to determine the weight increase of the treated steel plate and the weight reduction of the eluminum coil for the purpose of determining the electrolytic current efiiciency. The results are shown in the following table:

Even with a slight formation of trees, the deposited aluminum was liable to be scaled off in the course of the succeeding water cleaning step, thus the apparent current efficiency actually became still lower and in a suddenly decreasing manner. When the anodic current efficiency was measured relative to the dissolved-out quantity of aluminum, it amounted frequently to more than 100 percent, thus the aluminum content of the bath increased with the duration of the electrolytic treatment. It was observed that aluminum once deposited on the cathode dissolved out again, as evidenced by reduction of weight of the aluminum wire.

On the other hand, likewise in the above experiment, when the aluminum wire coil was kept in its submerged state in the molten bath at 160 C. for 30 days, a further dissolving-out of the aluminum could not be observed. When the bath temperature was reduced to 150 C., air bubbles entering the bath were observed to be accomplished on the marginal surface thereof by separated aluminum in the form of thin scales in crystalline state, which is also a sign of the presence of the said superfluous solute of aluminum.

EXAMPLE 4.

Using a molten bath comprising 62 mole percent aluminum chloride and 38 mole percent sodium chloride, but without any addition of conventional metal mist suppressing agent such as potassium chloride so as to clearly observe the aforementioned adverse effects by the presence of superfluous aluminum solute as set forth in the foregoing example 3, the electrolytic treatment was carried out at 160 C. for about 10 minutes with a cathodic current density of 2 a./dm. taking an aluminum plate as anode and a steel sheet as cathode, having the same dimensions as given above, respectively, the voltage being 0.3-0.4 volt and the treatment being repeatedly performed from several times to about 20 times per day and for an extended time period, such as 10 months. On the first day, the current efficiency amounted to 85-90 percent which decreased however to 62-70 percent on the e third day. On the l0the day, the efficiency decreased to as low as 10-20 percent. This made it very difficult to carry out the electrolytic coating process with satisfactory quality of the coating produced.

When, however, gaseous hydrogen chloride was supplied to the molten bath on the third day at a rate of cc./min. per one liter of the bath material and for about an hour, so as to provide hydrogen ions to the electroiytically reacting zone, the current efficiency was restored to 80 percent from the preceding value of 62-70 percent. When the duration of the supply of gaseous hydrogen chloride was further extended for 1.5 hours, the efficiency rose to percent which high value could be maintained for as long as 6 months by adopting the blowing-in operation of hydrogen chloride while continuing the electrolytic coating operation.

EXAMPLE 5.

A steel sheet stock, having same dimensions as set forth in example 3, after degreased, pickled and well water-cleaned, was treated as cathode in an aqueous solution containing 1 percent of HCl and 0.005 percent of PbClat a current density of l a./dm. for about 10 seconds, using a carbon electrode, as anode, having same dimensions as above. The stock was then water-cleaned to a satisfying degree. In this case, the stock should preferably be subjected to the influence of mechanical vibration as was already referred to hereinbefore, preferably at several to about cycles per second.

By the water-cleaning, soluble salts attached to the steel surface were substantially removed therefrom, and then stock was dried.

Should the treating liquid contain a higher concentration of PbCl than the above-mentioned value or 0.005 percent, the treated steel surface represented, to a slight degree, brown to gray color tone, yet demonstrating the natural glazing color of 2 On the other hand, when the PbCl content was lower than 0.00l0 percent, the appearance of the treated steel surface was substantially unchanged.

The thus treated stock was then subjected to a conventional electrolytic treatment in a molten salt bath comprising 58 mole percent of AlCl and 42 mole percent of NaCl according to the conventional technique for the formation of aluminum coating, providing thus a smooth, glaxing and well-bonded aluminum film on the stock and capable of being subjected, without any breakage of the coating, to a succeeding mechanical working for the fabrication of finished products. The thickness of the aluminum could be adjusted with ease to a value thicker than 10 microns which means a considerable progress in the art.

EXAMPLE 6 ty being 1 a./dm, the preliminary treatment was carried out.

88 about 30 seconds, further conditions for the preliminary step being the same as in the case of7 5.

The pretreated stock was further treated electrolytically with a current density of 2a./dm. as in the preceding example 5 ,thus providing an aluminum coating, l7.4p.thick, with a current efficiency of 88percent. The coating was beautiful, of metal glazing tone, and well-bonded with superior quality.

EXAMPLE 7 In this experiment, the content of PbCl, was adjusted to 0.0006 percent and the concentration of HCl was 0.3N, the current density being 1 aJdmF. The preliminary treatment was carried out for about 30 seconds, further conditions for the preliminary step being the same as in the case of example 5.

The thus treated test piece was further treated electrolytically with a current density of l a./dm in a bath which had been highly deteriorated that only a degreased and pickled stock could not treated therewith effectively, other conditions being similar to those as employed in example 5. The thus formed aluminum coating was beautiful in its metal glazing tone, and well-bonded with superior quality.

EXAMPLE 8 In this case, the aqueous preliminary treating bath contained 0.1 percent of bismuth oxide and. 1.0 percent of perchloric acid. The treatment was carried out with a current density of l a./dmF for about 30 seconds.

Then, the treated stock was subjected further to an electrolytic treatment while using the same molten salt bath as was set forth in example 7, and using a current density of 1 a./dm. Superior results in the formed aluminum coating were obtained. ln this case, however, the coating thus produced should be exposed to the open atmosphere at least once in the course of the next succeeding water-cleaning step, so as to turn the produced coating into inactivated one.

EXAMPLE 9 In this case, an aqueous solution of hydrochloric acid having a concentration of 1N and containing 0.05 percent of stannous chloride was used as preliminarily treating liquor. The treatment was carried out with a current density of l a./dm. for about 30 seconds.

Then, the treated stock was subjected further to an electrolytic treatment while using the same molten bath as was set forth in example 7, and with a current density of l a./dm. again. Aluminum film as coated on the stock with superior results.

EXAMPLE l0 ln this case, the preliminarily treating aqueous liquor contained 0.5 percent of lead acetate. The treatment was carried out with a current density of0.3 a./dm. for about 2 minutes.

The thus treated stock was then subjected to the second treatment as before for the coating it with aluminum, yet while using the same bath composition as set forth in example 6. The coating was slightly inferior in its characteristics, yet having a superior metallic glaze.

EXAMPLE 1 l The preliminarily treatment, was carried out with use of an aqueous hydrochloric acid solution, 2N, containing 0.5 percent oi'titanium chloride and at a current density of 20 a./dm.

Then, the thus treated stock was further subjected to an electrolytic coating treatment while using same molten salt as set forth in example 7. The current density was 1 a./dm. as before. The thus obtained aluminum coating had a superior metal glaze, providing in addition, superior tightness, smoothness and bond.

EXAMPLE 12 A steel sheet stock was preliminarily coated with a thin film of acetone of percent, purity, by dipping the stock therein.

Then, the treated stock was subjected to an ordinary electrolytic treatment, using a used-up molten bath comprising 58 mole percent AlCl and 42 mole percent NaCl, and kept at C. This bath had so deteriorated as to be considered unusable for conventional operation. The electrolysis was carried out with a current density of 1.5 a,/dm. for about 10 minutes. The aluminum coating which was obtained was completely smooth, tight, well-bonded and highly durable to scratching.

The acetone was replaced in succession by methyl alcohol, ethyl alcohol of 96 percent purity; ethylene glycol, 95 percent; glycerin, 95 percent; lactic acid, 87 percent; and formic acid, 87 percent. The results were similar.

EXAMPLE 13 An aqueous syrup (concentration: 1 percent) made of starch was used in place of the acetone used in example 12, and the treating conditions were the same as set forth in that example. Similar superior results were obtained.

When the starch was replaced in succession by sugar (concentration: 10 percent), casein 2 percent; egg albumen, l percent; gelatin, 1 percent; and water glass, 0.5 percent, similar superior results were obtained.

EXAMPLE 15 A dilute aqueous solution of common salt (concentration: 5%) was used and other treating conditions were the same as set forth in example 13. Similar superior aluminum coatings were obtained.

When the common salt was replaced in succession by magnesium chloride, 5 percent; potassium chloride, 5 percent; calcium sulfate, 0.2 percent; magnesium perchlorate, 0.5 percent, caustic soda, 1 percent; calcium chloride, 5 percent; aluminum chloride, 5 percent; barium chloride, 2 percent; zinc chloride, 2 percent; and chromium chloride, 1 percent, substantially the same superior results were obtained.

The dipped metal stock was dried at normal temperature and then left in the open air so as to absorb a certain quantity of water content from the ambient air, so as to form a thin aqueous film on the stock surface.

When any one of the above chemicals was replaced by a small amount of conventional surface active agent such as an alkylated sodium benzenesulfonate, or added in addition, similar results were obtained.

EXAMPLE 15 A combination of two or more of the substances referred to in the foregoing examples 12-14 could be employed with equal results when other operating conditions were similar to those set forth therein.

For instance, ethyl alcohol or an aqueous solution thereof was added with a small amount of TiCl, and coated in a thin film on a steel or other metallic sheet stock and the latter was treated under similar conditions as before. The resulting aluminum coating was highly superior due to the combined action of both chemicals. In this case, the stock after being dipped, was treated while in its wet condition. Even when the preliminarily treated stock was dried in a hot air drier and then treated as before, the nature and conditions of the resulting aluminum coating were substantially the same as before.

Although steel stocks have only been described hereinbefore, other various metallic stocks such as copper and the like may equally be treated by the inventive process. This will apply also to other metals than steel.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic aspects of this invention, and therefore, such adaptation should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. ln the process for electrolytically coating a metallic surface with aluminum in a molten salt bath containing aluminum chloride, wherein an aluminum metal member is used as an anode and the metallic surface to be coated as a cathode, wherein, as the electrolytic process proceeds, hydrogen ions are supplied into the bath from an external source in the form of small bubbles for interaction with the aluminum ions to improve the coating of the metallic surface, and wherein the source of hydrogen ion is hydrogen or hydrogen chloride, the improvement wherein said metallic surface is electroplated with a metal having a higher hydrogen overvoltage than that for aluminum to deposit a metal coating thereon prior to plating aluminum thereon.

2. The process of claim I, wherein the metal is selected from the group consisting of Pb, Sn, Ti, Zn, 80, Cr, and Cd.

3. The process of claim 1, wherein the metal is Pb.

4. The process of claim 1, wherein the metal is Bi.

5. The process of claim 1, wherein the metal is Sn.

6. The process of claim 1, wherein the metal is Ti.

7. The process of claim 1, wherein the metallic surface is a steel.

i i 1 i 

2. The process of claim 1, wherein the metal is selected from the group consisting of Pb, Sn, Ti, Zn, Bi, Cr, and Cd.
 3. The process of claim 1, wherein the metal is Pb.
 4. The process of claim 1, wherein the metal is Bi.
 5. The process of claim 1, wherein the metal is Sn.
 6. The process of claim 1, wherein the metal is Ti.
 7. The process of claim 1, wherein the metallic surface is a steel. 